Heater works part time

I have a 1996 318ti and the heater only works part of the time. After the car gets warmed up and I am moving down the road, it will work sometimes. When I come to a stop light, it will start to blow cold air again. Any ideas?

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Humm I make the assumption that you don't have overheating problems. Yes, sounds like a thermostat is stuck open. If you have a gage on the dash you may also notice that the engine temp takes a long time also. Sometimes when you sit a light for a while the temp edges up and starts to get warm....then when moving down the road a higher speeds the temp cools off again. this does this because the thermostat may be stuck open and it cools the heated water in the engine when moving. Also, check the coolant level in the radiator. (only do this when cold!!!!!!!!) ok????? NOT HOT! low coolant can affect how the heater performs. (low volume)

Just finished fixing my daughters 1999 taurus heat. It would blow warm only when running down the road. At idle it blew cold. I started with replacing thermostat, and water pump because they are the easier fix. Still did not help. Then I replaced the heater core. A daunting job but not that hard to move the dash slighly out of the way to replace. There is a good write up at tarusclub.com. After that, heat was better but at idle, it would still blow cool sometimes. Researched and found answer. Park car on the steepest hill you can find with front of car up. Start car and take off reservoir cap. Let car warm up and keep rpms about 1500. After a while the fluid will come running out of the car and all over the place but it will bring air bubbles with it. Reduce rpm, add more fluid and repeat about 3 times. It will make a mess but after that, the heater works perfectly. We get heat long before the temp gage indicates any and it never blows cold unless you lower the temp.